When it is intended for the instrument panel of an aircraft, but also for other fields, a keypad must satisfy a certain number of requirements, in particular dimensional requirements. A first requirement is the tolerance relating to the key overshoot. The key overshoot is the difference in height between the top surface of the key and the fixed surface of the keypad. This tolerance is usually slim, of the order of two to three tenths of a millimeter. A second requirement relates to the travel of the key. This travel must usually be between seven and ten tenths of a millimeter depending on the application. A third requirement relates to the force to be applied to a key in order to actuate the switch. The force is for example five or six newtons with a tolerance of about one newton. A fourth requirement may also relate to satisfaction from operating the switch, in other words to the tactile sensation obtained when pressing a key. This sensation is notably associated with the resistance put up by the key when it is pushed and with the marked change in resistance observed when the switch passes from the open state to the closed state. This sensation is important in ensuring reliable feedback to the operator operating the switch.
The dimensional requirements may be all the more difficult to satisfy because the fixed portion of the keypad is often made of an assembly of parts. An assembly is for example necessary when the keypad is backlit. The keypad then comprises at least one front face, a printed circuit forming a base and a diffuser interposed between the front face and the printed circuit. The keypad may also comprise sealing elements between the fixed portion and the movable portion, that is to say the key or keys. The assembly of various parts of the keypad causes a geometric dispersion usually of the same order of magnitude as the travel of the key and as the tolerance concerning the key overshoot. For a keypad designed for an aircraft instrument panel, the geometric dispersion is ordinarily between six and ten tenths of a millimeter depending on the tolerance of the parts and the care applied to assembling the keypad.
Moreover, push-button switches have an insufficient travel to achieve the minimum travel required for the key. In this instance, the travel of a dome switch is rarely greater than three tenths of a millimeter. Even for a switch incorporating elastomers, the travel is usually less than seven tenths of a millimeter. Consequently, it is not usually possible to produce a rigid connection between the key and the movable portion of the switch.